Detection of defects during production of articles, such as coated films or webs, allows the operator to take prompt corrective action to maximize quality. Fast corrective action is particularly important in the production of continuous articles. Removal or correction of defects in continuous articles can be difficult, and it is often economically prohibitive to separate sections with defects from the defect-free sections of that article. Defects in the center of a wound web or film may require a laborious secondary process to remove the defective sections and to splice the non-defective sections back together. If the web or film is later converted into discrete products, high converting waste can result. In either case, removal of defective sections is often not cost-effective and can result in the waste and disposal of the entire wound web or film.
Effective detection of defects can allow the operator to quickly mark the defects for the secondary removal process. Also, the operator can quickly adjust the primary process in which the defect originates to eliminate the cause of the defect and to minimize defects to an acceptable number. Additionally, detection of "pre-defect" symptoms can prompt the operator to adjust the process to avoid defects prior to their formation. The operator taking the corrective action can be human or automatic.
Apparatus to detect defects are well known. One type of defect-detecting apparatus is an image analyzer which compares an image of the article being inspected with either another image of the article or a programmed image. When the images contrast, the apparatus may consider the irregularity a defect. This type of apparatus can be very expensive depending on the sensitivity required and the type of defects being detected.
Another type of defect-detecting apparatus is the line scanning system. This system includes a laser beam, for example, which is repeatedly directed across the article being inspected. One embodiment of this system includes a photodetector which measures the intensity level of the beam after the beam passes through the article. When the beam is deflected by the article, its intensity is reduced and detected by the system. However, determining changes in intensity resulting from slight deflections is often difficult.
Another known embodiment of this system, rather than measuring intensity levels, includes a scanning beam, which is directed at the surface of the article at a small or zero-degree angle of incidence, and a photodetector having a limited collecting area. A small angle of incidence means that the beam is directed nearly perpendicular to the surface of the article. Directing the beam at a small angle of incidence is used when the beam reflected from the surfaces is also being collected in order to direct the reflected beam away from the source and toward the photodetector. Minimizing this angle is important so that the inspection system takes up the least amount of space.
With this embodiment, when the laser beam travels through the article without being deflected by a defect, the laser beam is collected by the photodetector. If the beam strikes a defect which deflects the beam outside of the collecting area of the photodetector, the failure to collect the beam is sensed by the photodetector.
However, when the beam is deflected by a defect such that the beam still strikes the photodetector, the defect will not be sensed by this system. For example, the deflection due to a defect may be so slight that the beam still strikes a portion of the photodetector. Likewise, if a beam, which when not deflected by a defect strikes one edge of the photodetector, but, in fact, is deflected to just within the other edge of the photodetector, the system will not sense that defect.
It is also known that this type of laser scanning system could be arranged differently so that, rather than collecting an undeflected beam, the undeflected beam is stopped by a masking component. With this arrangement, the photodetector is positioned to collect the beam if the beam strikes a defect which deflects the beam outside of the area of the masking component and within the area of the photodetector. Again, the beam can be deflected by a defect and still be stopped by the masking component causing the system to fail to detect the defect. Consequently, this type of scanning system has a limited sensitivity to defects or variations which cause only slight beam deflection.
In addition, a system of this type which directs a laser across the entire surface of the article and generally perpendicular to that surface uses large optical elements, such as mirrors, lenses, or collectors. The cost of the optical elements required to scan a wide web in this way is significant, and cost-prohibitive for many applications.
Also known is a masking component made up of multiple perforated layers, called a Moire Deflectometer. The perforations of each layer are aligned with the perforations of the other layers such that a beam deflected to a different location within the area of the mask can pass through and strike the photodetector. However, the alignment requires a high degree of accuracy to function and is susceptible to even slight vibration of the article being inspected.
There is a need for a cost-effective scanning system for detecting certain defects in wide articles which cause small deflections of a scanning beam.